-3.3 Activity Hazard Analyses (AHAs)

por Michael Lombardi 02 de junio, 2025

3.3 Activity Hazard Analyses (AHAs)

The following charts reference activity hazards associated with general closed circuit rebreather diving, coupled with recommended controls. The diver must understand the root cause of each hazard to properly manage the life support equipment subsystem and mitigate atmospheric management problems.

3.31 Controls for Rebreather Failure Modes

AHAs should be reviewed by the dive team and mechanisms to control the various activity hazards should be discussed. Recommended controls to mitigate risks associated with each activity hazard are presented, and you will note that these controls have substantially influenced the Principles of Design applied to our system logic throughout this content. When considering the adoption of rebreather technology into one's diving regimen, or even developing rebreather systems, controlling for the cited risks should become a priority for equipment selection and its configuration - remember - what we dive should follow how we dive, in response to mitigating encountered risks.

Activity	Potential Hazards	Recommended Controls
Rebreather Failure Modes and Recommended Controls
scrubber failure	hypercapnea/hypercarbia	Manufacturer scrubber durations will be strictly adhered to. Fresh absorbent will be used on all dives exceeding 40m.
scrubber flood	caustic cocktail	Pre-dive checks will be conducted to ensure water-tight integrity of breathing loop.
total electronics failure	unknown breathing mix	Diver will carry sufficient open-circuit bailout to abort dive and complete decompression. CCR teams will ensure bailout gas supplies are cross-compatible between units.
oxygen sensor failure	unknown breathing mix	CCR electronics will include 3 cells and/or provide a means to validate sensors using known diluent supply. If cells cannot be validated, diver will open-circuit bailout.
total loop failure	loss of rebreather life support	Diver will carry sufficient open-circuit bailout to abort dive and complete decompression. CCR teams will ensure bailout gas supplies are cross-compatible between units.
oxygen MAV or needle valve fails open - BOOM	spike in oxygen - hyperoxia	Diver will have means to dilute breathing loop, and isolate oxygen supply if necessary.
diluent MAV or ADV fails open - BOOM	drop in oxygen - hypoxia	Diver will have the means to manually add oxygen.
primary decompression gas source compromised	out of gas, reduction of required gas	Divers will carry redundant decompression gas in personal or team configuration. Redundant surface supplied breathing gas may be supplied via a trapeze. Redundant gas may be staged throughout the dive.
	forced to utilize alternate decompression gas	Divers will carry back-up computer or table and prepare for lengthened decompression time.
Rebreather Human Health Hazards
in-water injury - physiological	hypercapnea	Diver has immediately available open circuit gas supply that is breathable for given depth.
	hyperoxia	Diver will stop or slow descent (if descending). Diver will have immediately breathable open circuit gas source ready and available. Diver should verify that proper diluent is feeding breathing loop. Diver will verify that solenoid/leaky valve and/or O2 MAV are not stuck 'open'. Diver will take action to decrease inspired pO2.
	hypoxia	Diver has breathable open circuit gas supply ready and immediately available. Diver will verify that solenoid or leaky valve is functioning. Diver will verify that oxygen MAV is working. Diver will verify that ADS is not leaking hypoxic gas. Diver will take action to increase inspired pO2.
in-water injury - psychological	diver stress	Encourage calm with dive partner. Safety diver will deploy to encourage calm and deliver extra gas if needed due to elevated consumption. Diver will abort dive if stress is elevated on initial descent. Diver will abort work if stress impedes performance.
*NOTE for CCRs: All CCR divers will have an immediate access to a breathable open-circuit bailout gas supply appropriate for complete open-circuit bailout from the furthest point of entry/depth.

3.32 Controls for Operational Hazards

AHA’s are frequently developed for working/commercial diving operations, but too often overlooked within the scientific or exploration diving regimen and almost never addressed when sport diving. These should reflect the stepwise activities for the work/mission to be conducted with a thorough assessment for each step. Through my own work, I have evolved the following AHAs to reflect the standardized 4-phase approach to extended range decompression diving published in Lombardi & Godfrey (2011). These AHAs continue to evolve as we better understand each dive phase and identify mechanisms to mitigate operational risks.

By partitioning the dive profile into four phases, specific hazards for each dive phase can be further identified and further mitigated through operational procedures or even equipment modifications. The following tables identify hazards for each of these four phases, with recommended controls.

Activity	Potential Hazards	Recommended Controls
Additional Operational Hazards for Phase 1 (initial descent and final ascent, 20fsw to surface)
in-water injury - physiological	DCS, DCI (on final ascent only)	Establish procedure for 'experimental' in-water recompression or omitted decompression procedures.
	hypercapnea	Diver has breathable (at <20fsw) OC gas source ready and immediately available.
	hyperoxia (end of dive/final ascent only)	Take action to reduce inspired pO2, take air breaks.
	hypoxia	Diver has breathable (at <20fsw) OC gas source ready and immediately available. No hypoxic diluents will feed the system shallower than 20fsw.
Additional Operational Hazards for Phase 2 (precipitous descent)
in-water injury - physiological	spike in oxygen - hyperoxia	Diver will descend at a swift but consistent rate, providing for the breathing loop to normalize. Diluent will be utilized to make-up loop volume on descent. Diluent gas should be selected that can drop PO2 at maximum depth with a loop flush.
	HPNS	Slow descent to alleviate symptoms. Ascend to reduce pHe.
Additional Operational Hazards for Phase 3 (work phase)
too deep/too long	increased decompression obligation, insufficient or inappropriate OC bailout	Bailout plans identified pre-dive to account for too deep/too-long contingency. Limit surface-to-surface excursions to maximum too deep/too long contingency - ensure not to exceed primary life support and bailout capabilities. Designate one diver on in-water team as responsible for monitoring depth/time.
Additional Operational Hazards for Phase 4 (decompression)
in-water injury - physiological	Pressure related maladies-DCS, AGE, other barotraumas	Divers will ascend along a fixed point whenever possible, using the downline for reference during ascent. Dive computers will include ascent rate monitors and decompression status. Divers will use extra caution in managing variable volume equipment including buoyancy compensators, rebreather counterlungs, lift bags, etc. Divers will identify and prepare for emergency IWR or Omitted Decompression procedures.
	drop in oxygen - hypoxia	Divers will have an immediately breathable open-circuit gas supply available at all times. Hypoxic diluents will be isolated from the breathing loop in the shallows.
	oxygen toxicity-CNS/pulmonary	Dive plans will not exceed recommended daily and long-term doses for oxygen exposures. Any diver inadvertently exceeding 80% CNS will complete 5 minute air breaks after 80% is reached and until decompression is completed. Dive team will be familiar with in-water signs/symptoms of CNS toxicity and will cooperatively monitor team members.
omitted decompression or blown schedule	DCS/DCI	All divers will utilize THREE means for following a decompression schedule and exiting the water. These may include computers, custom-, or fixed tables. Dive teams will carry complimentary tables for redundancy amongst team members. Procedures will be well established for addressing omitted decompression.